The invention generally relates to the fabrication of massive Al-Mg alloy plate greater than 6 inches thick, and particularly such plate 8 inches thick or greater. All numbered alloy designations herein refer to Aluminum Association alloy designations, and all percentage compositions herein are weight per cent unless noted otherwise.
Heretofore, most commercially produced aluminum plate has been fabricated in thicknesses less than 5 inches. Conventional fabrication procedures generally comprised casting the alloy into a rolling ingot about 10-24 inches thick, homogenizing the ingot and then hot rolling the ingot to the desired thickness. When properly degassed and thermally treated, the quality of the plate produced by these procedures was generally excellent.
With the advent of large LNG tankers, such as described in U.S. Pat. No. 3,680,323, aluminum plate greater than .ident.inches thick was needed for the equatorial ring (interface structure 72 shown in FIG. 7 of the aforesaid patent). The ring is machined from the thick plate to the final shape.
The metal quality requirements for the equatorial ring are quite stringent because the entire tank and its contents are supported by this element which is critical to the basic design concept of leak-before-failure. Aluminum alloy 5083 was selected for this application because of its excellent weldability, excellent mechanical properties, such as strength, ductility, fracture toughness and fatigue crack growth rate and excellent cryogenic physical properties. The minimum tensile properties for 0 temper (full anneal) normally specified are as follows:
______________________________________ Long Short Direction Longitudinal Transverse Transverse Location* 1/4 t 1/4 t 1/2 t Units ksi kp/mm.sup.2 ksi kp/mm.sup.2 ksi kp/mm.sup.2 ______________________________________ Tensile 37.0 26.0 37.0 26.0 35.0 24.5 strength min. Yield 15.0 10.5 15.0 10.5 15.0 10.5 strength min. (0.2% offset) Elongation, 14% 12% 10% min. (2" or 4D) ______________________________________ *In accordance with ASTM provisions for tensile testing heavy plate
When fabricating the 5083-0 plate, greater than 5 inches thick, particularly greater than 6 inches, in accordance with conventional procedures, it was found that the one-fourth t properties in the longitudinal and long transverse directions readily met minimum requirements, but the one-half t short transverse (ST) properties could fail the minimum specified properties, particularly as to tensile strength and elongation. Occasionally, ST tensile strengths were as low as 30,000 psi and ST elongation was as low as 3%. The plate frequently failed to meet the Aluminum Association ultrasonic test requirements for Class C discontinuity limits, the least stringent class.
It was apparent that thick plate could be formed by casting an ingot directly to the desired plate size rather than trying to hot roll a thicker ingot down to the desired size. However, it was found that although the mechanical properties of "as-cast" 5083 ingot conventionally homogenized (i.e. 950.degree. for 24 hours) fully met all the minimum tensile requirements set forth above, including short transverse property requirements, butt weldments of conventionally homogenized cast-to-size 5083 plate had unacceptable properties. The weldments met minimum ASME tensile specifications, but they consistently failed to meet bend test requirements (31/3 t radius). Failure of the bend test indicates an unacceptable weldment.
It is against this background that the present invention was developed.